Integrated Raman Quantum Memories
Optical quantum information processing offers a very promising platform for quantum technologies because photons are essentially noise-free at room temperature, interact very weakly with the environment and have many degrees of freedom in which information can be encoded. However, generating single photons on demand, as well as inducing direct photon-photon interactions deterministically remain unsolved problems.
An optical quantum memory is a device that can coherently store a quantum state of light, and then later retrieve it on demand without degradation of photonic quantum information. Optical quantum memories could be used to synchronise probabilistic single photon sources using a temporal multiplexing strategy, and reduce the waiting time for the generation of many photons simultaneously by many orders of magnitude.
In Oxford we focus on Raman quantum memories, which use an off-resonant two-photon Raman transition to coherently map a signal onto the ground state coherence, or spin wave, of an ensemble of alkali atoms. They have advantages over other memory protocols of being very broadband, with the ability to store GHz-bandwidth pulses with μs lifetimes, and operating at room temperature. Recent results have shown the suppression of four-wave mixing noise by implementing the Raman memory in a low-finesse cavity, as well as high memory efficiencies of ~60%. We next plan to demonstrate high efficiency storage of single photons and retrieve them with a second order correlation g(2)<1, confirming the quantum nature of the memory for the first time. Moving on from this we will use the quantum memory to temporally multiplex a single-photon source to increase its brightness in a given time bin, and then multiplex multiple sources to enhance multi-photon rates.